1. Field of the Invention
The subject invention relates generally to phase locked crystal oscillators and more particularly to an oscillator implemented with digital logic elements.
2. Description of the Prior Art
Phase locked oscillators serve many useful purposes in communication equipment. One of them is to synchronize the receiving equipment to an incoming signal so that information carried by that signal can be reliably interpreted. One example of such a signal is a square wavetrain which is encoded with a coding scheme such as the Manchester Code, in such a manner that the information carried by the wavetrain is manifested by a change in the logic level of the signal relative to the logic level of an unmodulated reference wavetrain. Thus, for example, a binary zero may be represented by the logic level of the signal wavetrain having the same value as that of the reference wavetrain and a binary one might be represented by the logic level of the signal wavetrain being opposite that of the reference wavetrain. In order to reliably interpret the information carried by such an incoming signal wavetrain it is necessary to lock a local frequency in phase with it so that a change in the logic level of the incoming signal during any given one of its cycles can be related to the logic level of the corresponding cycle of the local frequency with which it is compared.
Oscillators of the above type are well known. However, the elements generally used to lock their phase to that of a reference signal have conventionally been analog in nature, resulting in the generation of an analog error correction signal, requiring that it be filtered and conditioned before it might be used to control an analog correction device such as a tuning diode which then might in turn control the crystal oscillator phase or frequency. In applications where the local crystal oscillator is to be used with digital components, such as gate arrays, it would be a decided advantage to be able to use such components in place of the analog filtering and conditioning circuitry and the tuning diode.
It is therefore the principal object of the present invention to provide a quartz crystal oscillator whose phase can be accurately locked onto a reference wavetrain by the use of digital circuit elements.